Diarrheal disease is the second most common cause of death among children under five years of age globally, and infectious diarrhea is the second leading cause of global morbidity. Vibrio cholerae causes severe secretory diarrhea in humans, and is a prototypical mucosal infection that does not invade the intestinal epithelium;V. cholerae infection thus serves as an excellent model for the study of mucosal immunity and vaccination. Unfortunately, protective immunity following cholera is not currently understood, and available cholera vaccines either fail to produce full protective efficacy or induce less than optimal and relatively short-lived immune responses that fall to baseline within 6-36 months of vaccination. This is in comparison to natural infection with V. cholerae that induces protective immunity that lasts for 3-10 years. Serum vibriocidal and other serum antibody responses wane within 6-12 months of infection, suggesting that these current immunologic markers cannot be used as correlates of longer-term protective immunity. We have data to suggest that memory B cell responses to T-dependent protein antigens develop following V. cholerae infection and persist for at least one year, while memory B cell responses to a T-independent antigen, LPS, develop following cholera, but appear to wane by 9-12 months following infection. We have additional preliminary evidence of a CD4+ T helper cell response following V. cholerae infection, and we hypothesize that this response is necessary for the development and maintenance of B cell memory at the mucosal surface, that the T cell response may be qualitatively or quantitatively different between natural infection and cholera vaccination, and that these differences may explain the lessened efficacy of current vaccines for cholera and other mucosal infections. To address these questions, we propose five specific aims: (1) Characterize immune responses in blood following natural cholera, focusing on development and maintenance of memory B cell and T cell responses;(2) Evaluate mucosal innate and acquired immune responses following cholera using endoscopically obtained duodenal (EGD) samples, and correlate with responses seen in blood;(3) Assess innate and acquired immune responses early after exposure in household contacts to determine correlates of subsequent protective immunity to cholera;(4) Assess immune responses following cholera vaccination with the current killed oral rBS-WC cholera vaccine (synergizing with an on-going and separately funded cholera vaccine study), and compare responses to those following natural cholera;and (5) Evaluate host factors influencing susceptibility and immune responses to cholera. This proposal is built upon an on-going collaborative effort between researchers at the Massachusetts General Hospital-Harvard University and the ICDDR,B in Dhaka, Bangladesh. RELEVANCE: Cholera affects 5-7 million individuals each year, killing over 100,000, globally. Identification of protective immunity against cholera could not only directly affect cholera vaccination strategies, but could be applicable to the development of improved vaccines against other mucosal pathogens.